engine break in issue

I'll vote for an intake leak, as well.
I will add that milling the heads may have had some affect on intake seal, especially if the intake is a used aftermarket piece of unknown history.

I ALWAYS use the Blue Fel-Pro intake gaskets on the 340/360 smallblocks, (not sure if there is a 273/318 version) but they are a bit more forgiving of imperfections and, in a pinch, I have actually re-used them on intake swaps with no ill affects.

Mark.